Acoustic monitoring of two phase feed nozzles

ABSTRACT

Passive coustic method for monitoring and controlling flow state of two phase fluids through feed nozzles and thereby improving operating stability and high product value yields in major refinery processes or any other process where a finely atomized liquid stream exiting the nozzle is important.

The present invention relates to a process to monitor and control feedatomization in fluidized bed reactors or transfer lines. Petrochemicalprocesses that depend on fluidized beds or transfer lines include thethermal cracking of heavy oils in processes described as "fluid bedcoking" or "flexicoking", and the catalytic cracking of complexhydrocarbons in the process called "catalytic cracking" or "catcracking". Such processes are major components of modern refinerieswhich use them to convert more and more difficult feedstocks intopetroleum products of great added value.

One key component of either fluid bed coking or cat cracking is the feednozzle. Feed nozzles are designed to finely atomize heavy oil in orderto allow dispersion of a thin, uniform oil layer on approximately 150micron coke particles (fluid bed coking) or approximately 60 microncatalyst particles (cat cracking). Maintaining the performance of thesenozzles through the multi-year running cycle of a refinery is difficultfor very important to operating stability and high value product yields.However, maintaining the performance of these feed nozzles can becomplicated by several factors. For example, the oil feed to the feednozzles is typically very viscous and the composition of the oil is veryvariable. Under these conditions, small changes in the temperature ofthe oil feed can have a dramatic effect on the performance of thenozzle. Furthermore, the feed nozzle is inserted in a harsh environmentwhere erosion by particles and plugging by process deposits can bothadversely affect feed nozzle performance. In addition, the feed nozzlesusually receive feed and steam from manifolds that supply a multiplicityof feed nozzles. Under these circumstances, it is difficult, if notimpossible, to ensure that each nozzle is carrying a desired flowwithout monitoring the specific nozzle.

Of equal importance is the fact that the feed nozzles used in fluid bedcoking and cat cracking contain a two phase mixture of non-ideal fluidsnamely steam and heavy oil. Because of the non-ideal nature of thesemixtures, feed nozzles typically operate in a number of flow regimes.These range from the desired stable flow regime where the steam and oilare finely mixed to provide the desired atomization, up to theundesirable flow regime where steam and oil are alternately passedthrough the nozzle with very little mixing ("slugging").

Two of the major factors involved in obtaining a desired flowcharacteristic for a feed nozzle are: (1) the amount of steam injectedinto the feed nozzle with the heavy oil; and (2) the supply pressure ofthe heavy oil to the feed nozzle. Steam usage in heavy oil feed nozzlesperforms two functions. Firstly, it keeps the feed nozzle clear fromblockage when the feed oil is removed from the nozzle. Secondly, andmost importantly, it disperses the heavy oil feed into fine dropletsthat improve the contact between the feed oil and the coke or catalystparticles. However, since the steam introduces a highly compressiblecomponent into the oil, its presence has a major effect on thehydrodynamics of the steam/oil mixture and, therefore, introduces a highdegree of uncertainty into feed nozzle design. The volume of steam tovolume of oil ratio used in a particular nozzle design is a key factorin obtaining a desired feed nozzle flow characteristic. This steam tooil ratio can be affected by a number of different operating conditionsincluding feed viscosities. The supply pressure of the heavy oil feedcan have a major effect on the nozzle flow characteristics as well asflow instabilities of feed nozzles supplied by a common oil manifold.

In general, obtaining and maintaining designed feed atomization is adifficult task and one that is of major importance. In fluid bed cokers,poor atomization can lead to immediate local defluidization and theformation of large agglomerates which can reduce circulation. In theextreme, so many agglomerates are formed that the entire bed maycollapse. Poor feed atomization is also considered a cause of excessivebuild up of wall coke in cokers. Run limiting "upsets" of the processare often the result of the spalling of large chunks of coke which fallinto critical regions of the circulation system and disrupt the flow. Incat crackers feed atomization has a direct effect on process yield andproduct composition.

Until recently, ensuring that feed nozzles are operating in the desiredflow regime, and maintaining that condition under changes in feed andprocess conditions has been a matter of trial and error. For example,nozzles could be routinely "rodded out" or mechanically cleaned. Therehas been no way of verifying on an operating unit the flow regime of thenozzle since there was no direct tool to monitor flow. It has now beenfound, and this is the subject of this patent application, thatvibrational or dynamic pressure monitoring of feed nozzles (referred toin this patent application as passive acoustic nozzle monitoring) cangive quantitative information on the fluid state exiting the nozzle. Awell atomizing nozzle can be easily distinguished from one that isplugged, or that is exhibiting slug flow. Based on the technique ofpassive acoustic nozzle monitoring, operators of a fluid bed coker orcat cracker can take appropriate actions to restore the desiredoperating conditions. These actions could include changing the relativeratios of oil and steam, rodding or cleaning out the nozzle or isolatinga "bad nozzle" from the feed distribution system.

SUMMARY OF THE INVENTION

The present invention is a passive acoustic process whereby the currentoperating state of a feed nozzle injecting a mixture of liquid and gasinto a process vessel can be non-intrusively determined using thenaturally occurring energy in the nozzle to set up a recognizablesignature of vibrational resonances which can then be compared to thevibrational signature of the desired operating state of the feed nozzle.If there is a significant difference, corrective actions, such ascleaning the nozzle or changing the relative proportions of the liquidand gas, can be undertaken. The effect of such corrective actions torestore the nozzle to its desired operating conditions will be verifiedby repeating the same passive acoustic process. The process is a passiveacoustic process since it senses the naturally occurring vibrations thatare generated by the nozzle in its performance. In many commercialsituations of interest to the petrochemical industry the feed the nozzlewill be injecting is comprised of oil and steam. However, the inventioncan have wider application in any situation where it is important tomaintain specified flow conditions through an injecting nozzle carryinga gas-liquid mixture including particulate laden liquids such asslurries and where the reliability of the gas supply can be poor such asorifice limiting systems. It is particularly applicable to manifoldedsystems where the nozzles are fed by a common manifold and where thepiping to the nozzle is complex due to economic constraints. Under thesecircumstances it is difficult, if not impossible, to achieve a specifiedflow through each nozzle without individual monitoring and "tuning".

In a feed nozzle where oil and steam mix and are propelled out of thenozzle by the pressure drop across the nozzle, there can exist a varietyof pressure and vibratory resonances in the volumes defined by regionsof restricted or turbulent flow, including the nozzle tip. Theseresonances are concentrations of energy in the fluid or the structure ofthe nozzle in certain frequency ranges and result from the excitation ofthe acoustic and vibratory modes of the two phase fluid nozzle system.The multiplicity of resonances of variable intensity that occur over agiven frequency range with different magnitudes constitute a signaturein the power spectrum of the nozzle. The power spectrum itself can beobtained in a variety of ways. For example, in one embodiment of thispatent, we utilize spectral analysis of the electrical output of anaccelerometer in contact with the physical structure of the nozzle or ofthe electrical output of a dynamic pressure transducer in contact withthe two phase flow within the nozzle.

From the mathematical and vibrational literature, it is conventional todisplay in the power spectrum the mean square acceleration per unitfrequency range as a function of frequency; however, any mathematicalfunction of the mean square acceleration per unit frequency range can beutilized to obtain such a suitable power spectrum although somerepresentations will be found to be more convenient than others. As willbe demonstrated, there can be signatures of the nozzle operating statein the time domain as well. Either the power spectrum and its variationwith time or the time variation of the vibratory or dynamic pressureover a time interval can be used as a signature for monitoring the feednozzles. In what follows, both the frequency and time domaindescriptions will be discussed.

We have discovered that this power spectrum is stable in time as long asthe flow state of the fluid exiting the nozzle is constant and that itcan thus function as a two dimensional "signature" or "fingerprint" ofthe fluid state exiting the nozzle. Changes in the flow state of thenozzles brought about by changes in the nozzle tip or bore ("plugging")or by changes in the fluid inputs ("slugging") can be detected andcorrected by appropriate procedures to return the power spectrum to thatcorresponding to the desired flow state. Furthermore, the power spectrumcan be used to confirm that the corrective action has produced a returnto the desired flow state. Recognition of significant changes in thepower spectrum can be accomplished by a human observer in the case ofperiodic nozzle monitoring or by suitable pattern recognition algorithmsin the case of continuous or real time nozzle monitoring. Furthermore,the power spectrum can be obtained simply and nonintrusively while thenozzle is operating. Examples of the correlation between the powerspectrum of a particular nozzle and different flow conditions from thatnozzle will be discussed below. The frequency range of the powerspectrum is chosen so that resonances generated by the energy of theoperating noise of the nozzle dominate the power spectrum.

It is surprising indeed that it is possible to find a frequency rangewhere meaningful information can be obtained about nozzle flowconditions that is not masked by the background noise level of thereactor.

Under certain flow conditions additional information can be obtainedfrom the time variation of the vibratory signal. For example, the fluidexiting the nozzle can be regularly alternating its flow state (e.g.intermittent slugging or sputtering or just unsteady flow). This is anundesired state of flow and as such will exhibit itself in changes inthe power spectrum. The power spectrum is the fourier transform of thetime variation of vibrating signal. However, this undesired state offlow will also be a recognizable feature of this time variation of thevibrational signal and this time signature can be used to complement theinformation contained in the power spectrum. Again, examples of thecorrelation between certain time signatures and the flow states of thefluid exiting the nozzle will be given below.

The passive acoustic nozzle monitoring process includes the followingsteps:

1. A reference power spectrum (Reference Power Spectrum or RPS) isobtained from a vibrational sensor in close proximity to the nozzle orthe fluid contained within when the nozzle is atomizing feed in thedesired manner. This Reference Power Spectrum (RPS) is specific to anozzle of specified mechanical dimensions and fluid connections and hasto be determined empirically because of the well known complexity of twophase flow. Clearly under steady state conditions the RPS will notchange. In the present patent application the vibrational sensor iseither an accelerometer attached to the nozzle shell or a pressuretransducer in contact with the fluid within the nozzle. The frequencyrange over which the power spectrum is plotted is chosen empirically sothat the vibrational resonances that characterize the power spectrum aredominated by energy produced by nozzle flow.

2. Subsequent current power spectra (Current Power Spectrum or CPS) aretaken in either real time by a hard wired system monitored by a computeror taken periodically by operating personnel at the specific nozzle.

3. Comparison of the RPS with the CPS is made either by suitablecomputer pattern recognition algorithms or visually by personnel andchanges noted.

4. Changes in the operating conditions of a specific nozzle are thenmade to restore the CPS to the RPS.

5. The time variation of the vibratory signal that accompanies thetaking of both the RPS and the CPS can be noted and used either by asuitable computer algorithm or by observation of personnel to supplycomplementary information on the variation of the state of fluid flowwithin nozzle within the measurement time.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a schematic diagram of the system according to the methodof the claimed invention.

FIG. 2 shows a schematic figure of two nozzles being fed oil and steamthrough common headers.

FIGS. 3A, 3B, and 3C illustrate the conversion of a time varyingacceleration to a power spectrum: FIG. 3(A) shows the signal produced byaccelerometer as a function of time, FIG. 3(B) shows the square of thesignal produced by the accelerometer as a function of time and therelationship between the mean square acceleration and the area under thepower spectrum, and FIG. 3(C) shows the power spectrum as a function offrequency.

FIGS. 4a-h show the variation of the power spectrum as the flowconditions of a nozzle vary. The ordinate is in (volts)² /Hz. Theabscissa is in Hz. All pressures, P, are in psi.

FIGS. 5a-f are other examples of the variation of the power spectrum asthe flow conditions of the nozzle vary. The ordinate is in (volts)2/Hz.The abscissa is in Hz. All pressures, P, are in psi.

FIGS. 6a-f show the spectra of a nozzle obtained with a pressuretransducer for a variety of flow conditions. The ordinate is the log ofRMS pressure. The abscissa is in Hz from 0 to 32000 Hz.

FIGS. 7a and b show the time variation of the vibrating signal and thecorresponding power spectrum for the same nozzle conditions.

FIGS. 8a and b show the time variation of the vibrating signal fornormal oil flow and unstable oil flow.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The present invention gives a method for nonintrusively determining ifthe liquid-gas mixture exiting a nozzle has departed from desiredoperating conditions. If it has, then the operating conditions of thenozzle are changed to return to the desired flow conditions, or thenozzle is cleaned, or removed from service. The method of the presentinvention will be illustrated and described by a heavy feed fluid bedthermal conversion process such as in a fluid bed coker where the nozzlecontains feed oil and steam. However, it is not limited to petrochemicalapplications but to situations where it is difficult to directly monitorflow through a specific nozzle and where the fluid being carried or thenozzle environment leads to a high probability of unreliable nozzleperformance.

FIG. 1 shows a schematic diagram of a representative coker feed nozzle 4inserted through a coker wall 7. Fluid bed cokers can contain anywherefrom twenty to sixty such nozzles injecting feed at the rate ofapproximately ten to fifty gallons a minute. In one embodiment of theinvention, an accelerometer 5 is placed in close proximity to the nozzle4. In FIG. 4, the accelerometer 5 is placed on the rodding plug 6, butany location in proximity to the nozzle and where the accelerometer issensitive to changes in flow conditions is acceptable.

As shown in FIG. 2, for a given reactor, several nozzles 4 and 11 may beinterconnected so that steam and oil are supplied through a steam header14 and oil header 16 to all nozzles simultaneously. In this case, theperformance of one nozzle (upstream) can affect the power spectraobserved for another (discussed below).

The electrical signal for the accelerometer is proportional to thenozzle's vibration intensity and a plot of the mean square accelerationper unit frequency range or simple mathematical operations on thisquantity (e.g. taking the square root of this quantity and multiplyingby a constant to form a plot of the root mean square acceleration ortaking the logarithm of the quantity to form a "dB" plot) is anappropriate power spectrum for the purpose of this patent application.In one use the sensor signal is amplified and transmitted by anappropriate data link to a control room as shown in FIG. 1. In thecontrol room, the signal is processed by a spectrum analyzer or fastfourier transform signal processor. Also in the control room by anappropriate algorithm, the RPS is displayed and stored in memory.Similarly current power spectra or CPS are taken in real time and by asuitable pattern recognition algorithm compared to the RPS. Changesbetween the CPS and the RPS are brought to the attention of the operatorto indicate changes in the flow regime of a specific nozzle forsubsequent correction.

To assist the operator in interpreting the changes in the CPS from theRPS, a "dictionary" of characteristic CPS associated with specific flowconditions can be contained in memory and displayed to indicate thedirection of changes to be made. Again, the "dictionary" is feed nozzlespecific and has been generated empirically due to the well knowncomplexity of two phase flow in complex piping. The determination of adictionary is illustrated and described below.

In addition an analysis of the time variation of the real time signalfrom the accelerometer sensor (time signature) is used to detect thepresence of unstable flow during the measurement time.

Alternatively, the vibratory signal from the accelerometer can beprocessed at the nozzle by a portable spectrum analyzer operated by atechnician and changes in the CPS from the RPS noted and correctiveaction taken with respect to a nozzle can be taken immediately at thenozzle location.

In another embodiment of the patent application, the accelerometer 5 isreplaced by a dynamic pressure transducer 8 in contact with the fluid.In this case, the transducer penetrates the rodding plug to contact thefluid. Alternately, the transducer 8 may be located elsewhere along thenozzle (see FIG. 1). The frequency dependent pressure can be used togenerate a power spectrum to serve as an RPS or a CPS.

FIG. 3 exhibits the relationship between the time varying electricalsignal produced either by an accelerometer in contact with themechanical shell of a feed nozzle or a pressure transducer in contractwith the two phase mixture within the nozzle and the frequencydistribution of the vibrational energy generated by the nozzle in itsoperation. It also shows the square of that signal as a function of timeand frequency and exhibits the well known relationship between the areaunder the power spectrum and the mean square signal (mean squareacceleration or mean square pressure fluctuation when the transducersare correctly calibrated) produced by the vibrational sensor. Themathematical relationship between the power spectrum, S(F), theacceleration, A(T), mean square acceleration, A² (T), and time, T, isexpressed as follows: mean acceleration <A(T)>=0, <A² (T)>=2∫S(F)dF.

Determining of Dictionary for a Nozzle

A dictionary for a given nozzle is obtained by doing a power spectrumanalysis for different flow conditions. FIG. 4 shows a sequence of powerspectra of a nozzle for changes in flow conditions as induced by oilflow changes by valve (1) in FIG. 1 or steam flow changes by valve 2 inFIG. 1, or conditions of nozzles upstream on the same oil and steamheaders. shows a power spectrum for a

FIG. 4(a) nozzle where the oil and steam are set at normal pressure of175 but which exhibits unstable and undesirable flow. The pressure ismeasured at point 10 in FIG. The nozzle is then rodded and cleaned.FIGS. 4(b), 4(c), 4(d) and 4(e) show a power spectra of the nozzle asoil flow is reduced with some steam input until the oil flow is shutoff. The pressures, P, are 150, 130, 100 and 20, respectively. There isno chugging. FIG. 4(f) shows the power spectrum for the nozzle afternormal oil and steam flow are resumed. Pressure is 175 and there is nochugging. FIG. 4(g) shows the power spectrum for the nozzle with thesteam shut off and only oil flow. Pressure is 300. FIG. 4(h) shows thepower spectrum of the nozzle after normal oil and steam flow areresumed. The pressure is 175 and there is no chugging.

Nozzle malfunctions resulting from an inadequate mixing of feed andsteam may be correlated to the various open and closed valves aspresented discussed in FIG. 4. These include: partial or completeblockage of the nozzle due to deposits, oil off, steam off, improperproportions of steam to oil, fluctuating flow or chugging, as well aspoor atomization, time dependent shifts in flow between two nozzles onthe same feed ring as well as physically damaged nozzles. FIG. 5 isanother example of power spectra generated by an accelerometer incontact with the shell of the nozzle corresponding to such states. FIG.5 shows acceleration power spectra taken for a different feed nozzleunder different flow conditions. Pressure is measured at point 10 inFIG. 1.

FIG. 5(a) shows the power spectrum for the nozzle while it is plugged(no flow). FIG. 5(b) shows the power spectrum of the nozzle when it ischugging, showing unstable flow. The pressure is 183. FIG. 5(c) showsthe power spectrum of the nozzle after it has been cleared showingstable flow. The pressure is again 183. FIG. 5(d) shows the powerspectrum of the nozzle with only oil flow (steam off). Pressure is 300.FIG. 5(e) shows the power spectrum of the nozzle with only steam flow(oil off). Pressure is 50. FIG. 5(f) shows the power spectrum of thenozzle when another nozzle upstream on the same oil and steam headers isplugged. In this case, pressure is measured at point 12 in FIG. 1. It isimportant to note that despite differences between the power spectra ofFIGS. 4 and 5, each is a "fingerprint" of flow states.

An accelerometer will often be the vibrational transducer of choice forgeneration of the RPS and CPS. However, a pressure transducer is analternative that exhibits some advantages. It is often possible to placethe pressure transducer at a location where it can not be inadvertentlydamaged when cleaning a nozzle. There is no difference in the frequencycoverage between an accelerometer and a pressure transducer for thevibratory signals of interest to passive acoustic feed nozzlemonitoring. Furthermore, the efficiency of a pressure transducer incontact with the fluid is little affected by a build up of coker orother solids on its active surface since it is acoustic rather thanmechanical contact that it is important.

As discussed above, a dynamic pressure transducer (8) in contact withthe fluid within the feed nozzle at a fixed location on the feed nozzlemay be used to obtain a power spectrum. The power spectrum from such atransducer is shown in FIG. 6(a) through 6(f) show the power spectra ofa nozzle when it is operating correctly (6a), steam only (6b), samesteam, reduced oil (6c), oil, no steam (6d), induced slugging by addingsteam (6e) and nozzle instability induced by plugging up-stream nozzle(6f). A log scale is used for these figures to give greater dynamicrange.

From the above figures, it is clear that the pressure transducerproduces an equally distinct power spectrum to function as a CPS or RPSas does the accelerometer generated power spectrum. However, thepressure transducer generated power spectrum is simpler in that itcontains a smaller number of peaks and hence more changes in it are morereadily recognized by either a human observer in the case of periodicnozzle monitoring or by simpler computer pattern recognition algorithmin the case of continuous or real time nozzle monitoring.

The simplicity of the pressure transducer generated power spectrum canbe understood if we note that the peaks in the power spectrum arise fromresonances in the fluid contained in the bore or ancillary tubing of thefeed nozzle which have been excited by broadband noise from the fluidexiting the nozzle tip. The peaks in the power spectrum generated by theaccelerometer in contact with the nozzle shell contain as wellvibrational resonances associated with the mechanical vibrational modesof the nozzle as well as structural modes of the process vessel. Usingthe concept of acoustic impedance, it is easily shown that the signalsproduced by resonances in the fluid are favored over other resonanceswhen measured in the fluid by a factor of almost 100. Apart from thisfact, another advantage for the pressure transducer generated CPS isthat it may often be advantageous to use a pressure transducer forconvenience of location and protection from unintended damage duringnozzle maintenance.

As discussed above, the time variation of the vibrational signal alsomay be used to obtain information regarding the state of the flow, stateof fluid in the nozzle. FIG. 7(b) shows the time variation of thevibrating signal and the corresponding Fourier transform in FIG. 7(a),the power spectrum as a function of frequency for a chugging nozzle. Itis sometimes more convenient to use the time variation of the probesignal to detect unstable flow. FIG. 8 compares the time variation forstable oil flow (a) and unstable oil flow, chugging (b).

If the time variation of the vibrational signal is used to determine thestate of flow, then the vibrational signal over a later time period iscompared to the vibrational signal over an initial time period. Thelater time period may be continuous with the initial time period(sequential), or it may be separated from the initial time period by anintervening time period (discrete time periods).

What is claimed is:
 1. A passive acoustic nozzle monitoring process forcorrecting the operating conditions of a feed nozzle injecting a liquidgas mixture into a process vessel or chamber so as to maintain thecorrect mixing of said liquid and gas exiting the nozzle comprising:(a)determining an initial time variation of the vibrational signal over aninitial time period from a vibrational sensor in close proximity to saidnozzle when said nozzle is performing at a standard mixing or flowcondition which is the desired operating state of the nozzle; (b)determining a time variation of the vibrational signal over a later timeperiod from said vibrational sensor in close proximity to said nozzle;(c) comparing said initial time variation of said vibrational signalwith said time variation of said vibrational signal taken at a latertime, so as to determine a change in said vibrational signal; and (d)correcting the flow state of the nozzle by changing the variablescontrolling the relative volumes of gas and liquid entering the nozzleso as to return said nozzle to said desired operating state.
 2. Theprocess of claim 1 wherein said vibrational sensor is an accelerometer.3. The process of claim 1 wherein said vibrational sensor is a pressuretransducer.
 4. The process of claim 1 wherein said time variations ofsaid vibrational signals are taken over discrete time periods.
 5. Theprocess of claim 1 wherein said time variations of said vibrationalsignals are taken over sequential time periods.
 6. The process of claim1 further comprising the step of determining a time variation of saidvibrational signal for a time period after said step of correcting theflow state of the nozzle comparing it to said initial time variation ofsaid vibrational signal to verify that said nozzle has returned to itsdesired operating state.